i) Field of the Invention
The present invention relates to a new material made predominately of a metal oxide, for example titanium dioxide and having both a mesoporous structure and chirality that arise from the chiral nematic ordering of a mesoporous silica that is used as a template. The invention also relates to a composite of the metal oxide and the mesoporous silica template. Still further the invention relates to a process for producing the new material. In this invention, a precursor to a metal oxide such as titanium dioxide is polymerized inside of the pores of mesoporous chiral nematic silica to create the composite material of silica with metal oxide such as titanium dioxide in the pores. After removal of the silica template, a mesoporous metal oxide, for example titanium dioxide, is obtained that is iridescent and may be obtained as a film.
ii) Description of the Prior Art
Hard templating (also called nanocasting) has emerged as a powerful method for constructing new solid-state materials with periodic order.[1-10] Although silica can be prepared with a variety of periodic structures (e.g., lamellar, hexagonal, cubic) and pore sizes by aqueous condensation of a sol-gel precursor in the presence of a surfactant template,[11,12] there are many materials for which this method cannot be applied. By using porous silica as a hard template, diverse nanostructured materials may be obtained with a wide variety of compositions (e.g. carbon, polymers, noble metals, and metal oxides) after etching of the silica.[13-18] The thermal stability of silica also allows for the use of high temperature treatments to generate highly crystalline mesoporous products that may be difficult to obtain using other methods.
The hard templating approach has been used to synthesize novel mesoporous materials. Yue et al. recently reported the synthesis of mesoporous rutile and anatase TiO2 using SBA-15 silica as the hard template,[19,20] and other hard templates have been employed in the synthesis of nanostructured titania.[21] High surface area nanocrystalline TiO2 is of particular interest for applications such as dye sensitized solar cells,[22] photocatalysts,[23] gas sensors,[24,25] and batteries.[26]
The incorporation of high surface area anatase TiO2 into photonic structures is a further challenge that has recently garnered attention.[27-30] By using titania in colloidal crystals and inverse opals, the high refractive index of TiO2 (n=2.2 to 3.0) can allow the formation of a complete photonic bandgap in these materials.
A chiral mesoporous titania was recently described by Gedanken and co-workers, with the chirality arising from a chiral ligand that was utilized in the titanium precursor complex.31 While these titania materials appear to possess an imprint of the chiral ligand (as shown by enantioselective adsorption studies) they do not possess any long range chiral ordering (e.g., chiral nematic order), and the material was amorphous.
To date the synthesis of mesoporous materials has been mainly limited to the ordered pore structures obtained from the lyotropic liquid crystalline phases of surfactants and block copolymers. A new form of mesoporous silica with a chiral nematic pore structure templated by the lyotropic liquid crystalline phase of nanocrystalline cellulose (NCC) was recently reported.[32,33] Condensation of a silica precursor (e.g., Si(OMe)4) in the presence of NCC affords a composite material of SiO2 with NCC in a chiral nematic organization. Upon removal of the cellulose template, a porous silicate is obtained as a free-standing film that has a long range chiral nematic structure resulting in photonic properties. By varying the pitch of the chiral nematic composites, mesoporous materials with tunable photonic properties are obtained.